These studies aim to 1) elucidate those structural aspects of the prothrombin molecule affecting its ability to activate to thrombin, 2) delineate and compare the activation of prothrombin by intrinsic and extrinsic pathways, and 3) to determine some of the physiological and pathological mechanisms controlling the in vivo metabolic rate of human prothrombin. The studies will make use of purified normal human prothrombin, genetically abnormal prothrombin, and abnormal prothrombins induced by the administration of coumarin anticoagulants. Activation studies will employ radioactively-labeled prothrombins, and will be followed by biochemical and immunochemical methods. Metabolic studies in vivo will employ radiolabeled prothrombin. We shall continue studies of some of the factors involved in the genesis of antibodies to Factor VIII (Antihemophilic globulin) in patients with hemophilia and in some normal individuals. Several factors contributing to the tendency for a fraction of the hemophilic population to produce Factor VIII antibodies, will be studied, particularly lymphocytic responsiveness, histocompatability phenotypes, and other genetic factors. The character of the antibody population will be studied closely, after isolation by affinity chromatography. The immunochemical relationships between normal Factor VIII and the abnormal Factor VIII molecules present in hemophiliacs will be investigated by a variety of immunologic techniques.